1. Field of the Invention
The present invention relates in general to a grating-coupled waveguide (GCW) sensor and, in particular, to an optical interrogation system and method for using polarization-modulated light beams to interrogate a GCW sensor in order to determine whether or not a biological substance is located within a sensing region of the GCW sensor.
2. Description of Related Art
Grating-coupled waveguide (GCW) sensors are fast becoming the technology of choice to enable accurate label-free detection of a biological substance (e.g., cell, drug, chemical compound). This technology typically involves the use of a waveguide evanescent field to sense changes in the refractive index of a GCW sensor caused by the presence of a biological substance in a sensing region of the GCW sensor. To generate the evanescent field, an optical interrogation system is used which has a light source that couples a light beam into a waveguide of the GCW sensor. The optical interrogation system also includes a detector that receives a light beam coupled out from the waveguide that is analyzed to determine the effective refractive index of the waveguide. In determining the effective refractive index of the GCW sensor it should be understood that the light beam received by the detector had interacted with the waveguide under a resonant condition, where the wavevectors of a diffraction grating, incoming light beam, and guided mode all sum to zero. And, that this resonant condition occurs only for a specific wavelength and angle of the incoming light where changes in this angle or wavelength corresponds to changes in the effective refractive index of the waveguide caused by the presence of the biological substance in the sensing region of the GCW sensor. Thus, the optical interrogation system is used to sense a change in the effective index of the GCW sensor which enables one to determine whether or not a biological substance is located within the sensing region of the GCW sensor.
For this technology to be viable, one must have an optical interrogation system and in particular a detector capable of accurately monitoring the resonant angle, wavelength, or both. In particular, the optical interrogation system must emit a light beam that interacts with the GCW sensor, and must in turn receive the light beam coupled-out off the GCW sensor and process that light beam to detect in real time any changes in it's resonant angle and/or wavelength. While there are many approaches for accomplishing these tasks, each has unique implementation challenges, since the light beam output from the GCW sensor is relatively weak and there are multiple sources of noise that degrade this light beam especially in high-throughput screening applications.
GCW sensors are particularly attractive for use in high-throughput screening applications, where the absence of fluorescent tags and the possibility of reduced false-negatives would provide a large cost advantage. For this reason, the microplate has been targeted as the platform for such sensors, where 96 or 384 individual wells provide the high-throughput access demanded by the industry. In this application, the waveguide and diffraction grating of the GCW sensor are located in the bottom of each well; e.g., the diffraction grating may be stamped into the well bottom, and the waveguide subsequently grown on top of the diffraction grating. The wells themselves are typically composed of an optically transparent, low-birefringence, low-cost plastic that is typically several millimeters thick. To probe the GCW sensor in the well bottom while leaving the tops of the wells open for fluid handling, etc., the optical light beam is emitted into the bottom of the microplate and passes through the well plastic before striking the GCW sensor. One source of noise for this type of optical interrogation system is produced by the Fresnel reflection emanating from the bottom surface of each well. Due to the large number of wells, one ideally tries to design the GCW sensor to operate with incoming light beams near normal incidence. As a result, this spurious Fresnel reflection which acts as noise is often inextricably mixed with the light beam output from the GCW sensor that contains the desired information about the resonant angle and/or wavelength. In addition to the Fresnel reflection caused by the bottom surface of the microplate, the top surface of the waveguide inserts yet another Fresnel reflection into the output light beam that mingled with the light beam that propagated as a waveguide mode and exited the GCW sensor through the diffraction grating.
In addition to these direct optical noise sources, the traditional optical interrogation system is susceptible to other electrical and optical noises. For example, the wavelength or angle of the output light beam is often monitored with detectors such as charge-coupled device (CCD) cameras or spectrographs that observe the signal in a DC fashion. All of the DC electrical and optical (stray light) noise can impede the detection of the resonant angle and/or wavelength in the output light beam. Accordingly, there is a need for an optical interrogation system and method that can avoid the aforementioned problematical noise sources when interrogating one or more GCW sensors. This need and other needs are satisfied by the optical interrogation system, GCW sensor and method of the present invention.